Process for separating off nitrogen

ABSTRACT

A process is described for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification into a nitrogen-rich fraction and a methane-rich fraction, the methane-rich fraction, for the purpose of cold generation at a pressure as high as possible, is vaporized against the feed fraction which is to be cooled and superheated and the nitrogen-rich fraction is compressed at least occasionally and/or at least in part and is fed to the rectification as reflux stream. 
     According to the invention at least occasionally at least a substream ( 16 ) of the compressed (C) nitrogen-rich fraction ( 9 ′) is expanded (f) after condensation (E 1 ) thereof and, for the purpose of cold generation, is at least in part, preferably completely, vaporized (E 1 ).

The invention relates to a process for separating off a nitrogen-richfraction from a feed fraction containing essentially nitrogen andhydrocarbons, wherein the feed fraction is separated by rectificationinto a nitrogen-rich fraction and a methane-rich fraction, themethane-rich fraction, for the purpose of cold generation at a pressureas high as possible, is vaporized against the feed fraction which is tobe cooled and superheated and the nitrogen-rich fraction is compressedat least occasionally and/or at least in part and is fed to therectification as reflux stream.

A process of the type in question for separating off a nitrogen-richfraction from a feed fraction containing essentially nitrogen andhydrocarbons will be described hereinafter with reference to the processshown in FIG. 1.

Via line 1, the feed fraction which contains essentially nitrogen andhydrocarbons and was optionally subjected to a pretreatment such assulphur removal, carbon dioxide removal, drying, etc., is fed to a heatexchanger E1 and in this is cooled and partially condensed againstprocess streams which will be considered in more detail hereinafter. Vialine 1′ the partially condensed feed fraction is fed to a preseparationcolumn T1.

This preseparation column T1, together with the low-pressure column T2,forms a double column T1/T2. The separation columns T1 and T2 arethermally coupled via the condenser/reboiler E3.

Via line 2, a hydrocarbon-rich liquid fraction is taken off from thebottom of the preseparation column T1, subcooled in heat exchanger E2against process streams which will be considered in more detailhereinafter and subsequently via line 2′ and expansion valve a is fed tothe low-pressure column T2 in the lower region.

Via line 3, a liquid nitrogen-rich fraction is taken off from the upperregion of the preseparation column T1. A substream of this fraction ispassed via line 3′ as reflux to the preseparation column T1. Thenitrogen-rich fraction which is taken off via line 3 is subcooled inheat exchanger E2 and fed via line 3″ and expansion valve b to thelow-pressure column T2 above the feed-in point of the above describedmethane-rich fraction.

Via line 4, a nitrogen-rich gas fraction is taken off at the top of thelow-pressure column T2. The methane content thereof is typically lessthan 1% by volume. In the heat exchangers E2 and E1, the nitrogen-richfraction is subsequently warmed and super-heated before it is taken offvia line 4″ and either given off into the atmosphere or optionally fedto another use.

Via line 5, a methane-rich liquid fraction is taken off from the bottomof the low-pressure column T2, which methane-rich liquid fractioncomprises, in addition to methane, the higher hydrocarbons which arepresent in the feed fraction. The nitrogen content thereof is typicallyless than 5% by volume. The methane-rich fraction is pumped by means ofpump P to a pressure as high as possible—this is customarily between 5and 15 bar. In the heat exchanger E2, the methane-rich liquid fractionis warmed and optionally in part vaporized. Via line 5′, it issubsequently fed to heat exchanger E1 and in this, before it is takenoff from the process via line 5″, is completely vaporized andsuperheated against the feed fraction which is to be cooled.

Processes of the type in question for separating off a nitrogen-richfraction from a feed fraction containing essentially nitrogen andhydrocarbons are implemented in what are termed NRUs (Nitrogen RejectionUnits). Nitrogen is always separated off from nitrogen/hydrocarbonmixtures when an elevated nitrogen content impedes correct use of thenitrogen/hydrocarbon mixture. For instance, a nitrogen content, forexample, of greater than 5 mol % exceeds typical specifications ofnatural gas pipelines in which the nitrogen/hydrocarbon mixture istransported. Gas turbines also can only be operated up to a certainnitrogen content in the fuel gas.

Such NRUs are generally constructed as a central process unit similar toan air fractionator having a double column, as described, for example,with reference to FIG. 1.

If the nitrogen concentration of the feed fraction then falls below alimiting value—this, depending on the objective, is between 20 and 30%by volume—, sufficient fine purification (<1% by volume of methane) ofthe nitrogen-rich gas fraction taken off from the low-pressure column T2via line 4 is no longer possible, since thermodynamic limits are set forthe generation of reflux for columns T1 and T2. In particular inprocesses having a nitrogen content in the feed fraction which increaseswith time—for example in the case of oil extraction with pressuremaintenance by nitrogen (EOR=Enhanced Oil Recovery) in which the gasaccompanying the petroleum becomes richer and richer in nitrogen in thecourse of years—therefore a part of the nitrogen-rich (product) fractionto be taken off from the process via line 4″ is used as reflux medium.

For this, at least occasionally a substream of the nitrogen-richfraction which is fed to a single-stage or multistage compressor C vialine 9 is compressed at least to the pressure of the preseparationcolumn T1, consequently to a pressure between 20 and 50 bar. Thecompressed substream of the nitrogen-rich fraction is fed via lines 9′and 9″ through the heat exchangers E1 and E2 and in these cooled andpartially or completely condensed.

Via line 10 and expansion valve e and/or lines 11/12 and expansion valved, the compressed substream of the nitrogen-rich fraction can be fed asreflux stream to the preseparation column T1 and/or the low-pressurecolumn T2. Alternatively, the compressed substream can be added at leastin part via line 13 directly to the nitrogen-rich (product) fraction. Bymeans of this procedure the operating range of the double column T1/T2,with respect to the nitrogen content in the feed fraction, can beexpanded significantly in the direction of a low nitrogen content.

The compressor C is to date used exclusively for maintaining the purityof the nitrogen-rich gas fraction taken off via line 4 from thelow-pressure column T2 in the case of variable nitrogen content in thefeed fraction. A low nitrogen content in the feed fraction requires ahigher compressor performance than a medium nitrogen content. From acertain nitrogen limiting value in the feed fraction, however, theoperation of the compressor C is no longer necessary. A typicalobjective is to process a feed fraction having a nitrogen contentincreasing with time. This leads to the fact that the compressor C mustdeliver its full output at the start. With an increasing nitrogencontent in the feed fraction, the compressor performance can beincreasingly decreased. From a certain nitrogen concentration in thefeed fraction, the compressor is without function.

It is an object of the present invention to specify a process of thetype in question for separating off a nitrogen-rich fraction from a feedfraction containing essentially nitrogen and hydrocarbons, which processallows the compressor to be used to its capacity, independently of thenitrogen concentration in the feed fraction, in order to amortize theconsiderable capital costs associated with the compressor.

For solution of this object, a process of the type in question forseparating off a nitrogen-rich fraction from a feed fraction containingessentially nitrogen and hydro-carbons is proposed, which ischaracterized in that at least occasionally at least a substream of thecompressed nitrogen-rich fraction is expanded after condensation thereofand, for the purpose of cold generation, is at least in part, preferablycompletely, vaporized.

In this case the nitrogen-rich fraction is advantageously compressed toa pressure between 20 and 80 bar and after condensation thereof isexpanded to a pressure between 1 and 20 bar.

Corresponding to an advantageous embodiment of the process according tothe invention, at least occasionally at least a substream of thecompressed nitrogen-rich fraction after cooling thereof iscold-producingly expanded and, for the purpose of cold generation, is atleast in part, preferably completely, vaporized.

According to the invention, the described compressor C is now no longerexclusively used for the described application—generating one or morereflux streams—but is in addition used for cold generation.

The refrigeration power generated according to the invention isadvantageously used for being able to give off as liquid products thefractions obtained by rectification.

Further advantageous embodiments of the process according to theinvention for separating off a nitrogen-rich fraction from a feedfraction containing essentially nitrogen and hydrocarbons which aresubjects of the dependent claims are characterized in that,

-   -   the still incompletely vaporized methane-rich fraction is fed to        a circulation vessel, only the liquid fraction occurring in the        circulation vessel of the methane-rich fraction is partially        vaporized and fed back to the circulation vessel and the        completely vaporized top product of the circulation vessel is        superheated,    -   the methane content of the nitrogen-rich fraction obtained by        rectification is less than 1% by volume,    -   the nitrogen content of the methane-rich fraction obtained by        rectification is less than 5% by volume, and    -   provided that the feed fraction is separated by rectification in        a double column consisting of a preseparation column and a        low-pressure column, in the upper region of the preseparation        column, preferably above the top tray of the preseparation        column, a helium-rich fraction is taken off and expanded into        the low-pressure column, preferably into the top region of the        low-pressure column.

The process according to the invention for separating off anitrogen-rich fraction from a feed fraction containing essentiallynitrogen and hydrocarbons, and also further advantageous embodiment ofthe same which are subjects of the dependent claims, will be describedin more detail hereinafter with reference to the exemplary embodimentshown in FIG. 2.

In the description and explanation of the exemplary embodiment shown inFIG. 2, the process sections which have already been explained withreference to FIG. 1 will not be considered in detail again.

In contrast to the procedure shown in FIG. 1, in the procedure shown inFIG. 2, now, at least occasionally, a substream of the compressednitrogen-rich fraction 9″ which was condensed in heat exchanger E1 istaken off via line 16, expanded in valve f and fed via line 17 to thenitrogen-rich fraction in line 4′. Together with this, the expandedsubstream is vaporized at least in part, preferably completely, in heatexchanger E1 for the purpose of cold generation. After the nitrogen-richfraction in compressor C is compressed preferably to a pressure between20 and 80 bar, in valve f it is preferably expanded to a pressurebetween 1 and 20 bar.

Via line 15, in addition, at least occasionally, a substream of thecompressed nitrogen-rich fraction, after cooling thereof in heatexchanger E1, can be taken off and cold-producingly expanded inexpansion turbine X. The expanded substream is subsequently likewise fedto the nitrogen-rich fraction in line 4′ via line 15′ and warmed in heatexchanger E1 for the purpose of cold generation. By means of thisembodiment the additional refrigeration performance is increased.

Corresponding to a further advantageous embodiment of the processaccording to the invention, the substream of the compressednitrogen-rich fraction taken off via line 15 from heat exchanger E1 canbe expanded in the expansion turbine X at a higher pressure and warmedin a separate passage of the heat exchanger E1 and subsequently fed toan intermediate stage of the compressor C. This further improves theefficiency of cold generation.

By means of the procedure according to the invention, at least onesubstream of the methane-rich fraction which was taken off via line 5from the bottom of the low-pressure column T2 can be given off in theliquid state via line 20 and valve h. Alternatively or insupplementation thereto, a substream of the nitrogen-rich fraction canbe given off in the liquid state via line 18 and valve g.

Alternatively or in supplementation to taking off a methane-rich liquidfraction via line 20, the methane-rich fraction taken off via line 5from the bottom of the low-pressure column T2 can also be firstsubcooled in heat exchanger E2 and given off via line 21 and valve i.

The same applies to the nitrogen-rich liquid fraction given off via line18, which liquid fraction is likewise first subcooled in heat exchangerE2 and given off via line 22 and valve k.

The compressor C can now, independently of the nitrogen concentration inthe feed fraction, be used optimally at full capacity at each timepoint. In particular in the case of a nitrogen content increasing withtime in the feed fraction, the capital cost of the compressor does notbecome worthless in the long term but it meets the additionaleconomically useful object of integrated LNG and/or LIN production.

In the case of a low nitrogen content in the feed fraction, the possibleLNG and/or LIN production is smaller than in the case of a high nitrogencontent. The installed compressor performance is therefore selectedaccording to an optimized product selection over the lifetime of theplant.

In contrast to the procedure shown in FIG. 1, in the procedure shown inFIG. 2 the still incompletely vaporized methane-rich fraction which istaken off from the heat exchanger E2 via line 5′ is not fed directly tothe heat exchanger E1, but to a circulation vessel D. Only the liquidfraction of the methane-rich fraction which is fed to the heat exchangerE1 via line 6 and occurs in the circulation vessel D is in partvaporized in heat exchanger E1 and subsequently fed via line 6′ again tothe circulation vessel D. The completely vaporized methane-rich topproduct which is taken off via line 7 at the top of the circulationvessel D is subsequently superheated in heat exchanger E1 before it istaken off from the process via line 7′.

The process management of the methane-rich fraction within the heatexchanger E1 is defined in space in that the pathway is divided into avaporization section and a superheating section. The methane-richfraction is then vaporized exclusively in the section of the heatexchanger E1 which is connected via line 6 to the bottom of thecirculation vessel D.

The process management described makes possible safe and stablevaporization of the methane-rich (product) fraction even under variableoperating conditions such as, for example, alteration of the crude gasrate, the crude gas composition, the crude gas pressure and, also, inthe case of controller fluctuations. These circumstances result, forexample, in a very pronounced manner in oil extraction with maintenanceof pressure by nitrogen (EOR=Enhanced Oil Recovery) in which the gasaccompanying the petroleum becomes increasingly rich in nitrogen in thecourse of years.

Corresponding to a further advantageous embodiment of the processaccording to the invention, in the upper region of the preseparationcolumn T1, preferably above the top tray of the preseparation column T1,a helium-rich fraction 8 is taken off and expanded by means of the valvec into the low-pressure column T2, preferably into the top region of thelow-pressure column T2. This embodiment of the process according to theinvention, in the case of helium-containing feed fractions, has theadvantage that the inert gas helium can be ejected and the consequencesof operational variations or changes in the helium fraction in the feedfraction can be attenuated by the backwash in the low-pressure column T2and do not lead directly to contamination of the nitrogen-rich (product)fraction with an increased methane content.

1. Process for separating off a nitrogen-rich fraction from a feedfraction containing essentially nitrogen and hydrocarbons, wherein thefeed fraction is separated by rectification into a nitrogen-richfraction and a methane-rich fraction, the methane-rich fraction, for thepurpose of cold generation at a pressure as high as possible, isvaporized against the feed fraction which is to be cooled andsuperheated and the nitrogen-rich fraction is compressed at leastoccasionally and/or at least in part and is fed to the rectification asreflux stream, characterized in that, at least occasionally at least asubstream (16) of the compressed (C) nitrogen-rich fraction (9′) isexpanded (f) after condensation (E1) thereof and, for the purpose ofcold generation, is at least in part, preferably completely, vaporized(E1).
 2. Process according to claim 1, characterized in that thenitrogen-rich fraction (9) is compressed (C) to a pressure between 20and 80 bar and after condensation (E1) thereof is expanded (f) to apressure between 1 and 5 bar.
 3. Process according to claim 1,characterized in that at least occasionally at least a substream (15) ofthe compressed (C) nitrogen-rich fraction (9′) after cooling (E1)thereof is cold-producingly expanded (X), preferably expanded (X) to apressure between 10 and 20 bar, and optionally vaporized and warmed (E1)for the purpose of cold generation.
 4. Process according to claim 1,characterized in that the still incompletely vaporized methane-richfraction (5′) is fed to a circulation vessel (D), only the liquidfraction occurring in the circulation vessel (D) of the methane-richfraction (5′) is partially vaporized (E1) and fed back to thecirculation vessel (D) and the completely vaporized top product (7) ofthe circulation vessel (D) is superheated (E1).
 5. Process according toclaim 1, characterized in that the methane content of the nitrogen-richfraction (4-4″) obtained by rectification (T1/T2) is less than 1% byvolume.
 6. Process according to claim 1, characterized in that thenitrogen content of the methane-rich fraction (5) obtained byrectification (T1/T2) is less than 5% by volume.
 7. Process according toclaim 1, wherein the feed fraction is separated by rectification in adouble column consisting of a preseparation column and a low-pressurecolumn, characterized in that, in the upper region of the preseparationcolumn (T1), preferably above the top tray of the preseparation column(T1), a helium-rich fraction (8) is taken off and expanded (c) into thelow-pressure column (T2), preferably into the top region of thelow-pressure column (T2).